In recent years, an AC-input type brushless DC motor has been increasingly used as a motor for driving a fan of ventilating devices, and it replaces a conventional induction motor for the better efficiency. FIG. 11 shows an example of AC-input type brushless DC motors (refer to patent document 1.) Rectifier 101 rectifies an AC power supply and obtains a high DC voltage, and then DC-DC converter 110 converts the high DC voltage into a low DC voltage, which is applied to motor coils 103 and 104. Based on a signal supplied from a rotor position sensing circuit, switching elements 107 and 108 control electric currents running through motor coils 103 and 104.
Smoothing capacitors are placed before and after DC-DC converter 110. In this case, fairly large capacitive aluminum electrolytic capacitors 109 and 111, in which electrolyte is impregnated, are used as the smoothing capacitors. However, the aluminum electrolytic capacitor is changed its characteristics by an ambient temperature.
FIG. 12 shows a comparison between a brushless DC motor and an induction motor when they drive a fan. FIG. 13 shows air-volume-static-pressure characteristics in the case shown in FIG. 12. The induction motor, as shown in FIG. 12, works during the period between P1 and P2 along curve ST1 of speed-torque characteristics. Reference marks “L1” and “L2.” represent load curves of the fan at static pressure=0 and static pressure=max.
When this induction motor is replaced with a brushless DC motor of which speed-torque characteristics is shown as curve ST2, the maximum static pressure stays the same as that of the induction motor; however, the air volume at working point P3 during the static pressure=0 (zero) becomes rather greater. As a result, the motor produces greater noises and vibrations. To the contrary, when speed-torque characteristics goes along curve ST3, the air volume stays the same as that of the induction motor; however, the maximum static pressure at working point P4 during the static pressure=0 (zero) becomes rather lower. In such a case, if the wind outside the appliance changes a pressure loss, the air volume also changes greatly.
In order to overcome these problems discussed above, the speed-torque characteristics of the brushless DC motor to be used for driving the fan is preferably similar to that of the induction motor, and the torque preferably increases at the greater speed.
Patent Document 1: Unexamined Japanese Patent Application Publication No. 2003-284307